Our long-term goal is to advance cell-based treatments for severe temporal lobe epilepsy (TLE). The proposed work examines the mechanisms regulating synaptic integration of transplanted GABAergic interneurons transplanted into the dentate gyrus of the hippocampus in the adult nervous system of mice with pilocarpine-induced TLE. GABAergic interneuron grafts made into the adult dentate gyrus suppress temporal lobe seizures in TLE mice. Our working hypothesis is that the mechanism for seizure suppression is formation of inhibitory synapses by the transplanted neurons onto granule cells (GCs) born into the epileptic brain environment. To test this hypothesis we will compare seizure suppression and inhibitory synapse formation by transplants of GABAergic precursors derived from three sources: 1) the medial ganglionic eminence (MGE) of fetal mouse brain, 2) mouse embryonic stem cells (mESCs), and 3) mouse induced pluripotent stem cells (mIPS cells) in the following two Specific Aims: Aim 1: Identify the functionality and synaptic partners of fetal MGE-derived GABAergic interneurons transplanted into the dentate gyrus of TLE mice. This aim tests the hypothesis that fetal GABAergic interneuron transplants suppress seizures due to formation of inhibitory synapses onto GCs born into the epileptic brain environment. Aim 1.1: Evaluate seizure suppression by EEG in TLE mice with transplants of fetal basal forebrain cells. Aim 1.2: Measure inhibitory post-synaptic currents in retrovirally-labeled populations of GCs by patch-clamp electrophysiological recordings in hippocampal slices from TLE mice with fetal interneuron transplants. Aim 1.3: Compare the extent of axon outgrowth and inhibitory synapse formation by the fetal GABAergic interneuron transplants onto retrovirally-labeled GCs in TLE mice. Aim 2: Identify the functionality and synaptic partners of mouse pluripotent stem cell (mPSC)-derived GABAergic interneurons transplanted into the dentate gyrus of TLE mice. This aim tests the hypothesis that mPSC-derived GABAergic precursor transplants will suppress seizures due to formation of inhibitory synapses onto GCs born into the epileptic brain environment. Aim 2.1: Derive MGE-like basal forebrain cells in vitro from mESCs with an Nkx2.1 BAC reporter construct. Aim 2.2: Derive MGE-like basal forebrain cells in vitro from miPS cells with an Nkx2.1 BAC reporter construct. Aim 2.3: Evaluate seizure suppression by EEG in mice with transplants of mPSC-derived MGE-like basal forebrain cells. Aim 2.4: Measure inhibitory post-synaptic currents in retrovirally-labeled populations of GCs by patch-clamp electrophysiological recordings in hippocampal slices from mice with transplants of mPSC-derived MGE-like basal forebrain cells.